Green’s Dictionary of Slang

a tip, bribery, the money paid as a bribe.

1937–84		Partridge DSUE (1984) 292/1: from ca. 1780.
1953		G. Durrell Overloaded Ark 30: I dashed him two packets of cigarettes, and he trotted off [...] A little distance away I saw the council members close in on him and skilfully relieve him of most of the dash [ibid.] 31: I will pay them one and six a day, and they will get dash for every animal they catch.
1981		W. Boyd ‘The Coup’ in On the Yankee Station (1982) 158: He said good-bye to Isaac, and Moses his cook [...] He’d given them all a sizeable farewell dash the previous evening.
1989		J. Morton Lowspeak 47: Dash – originally a gratuity but now a bribe.